1. Field of the Invention
The invention relates to a Sensitive Field Effect Transistor apparatus, particularly to an Ion Sensitive Field Effect Transistor/Reference Field Effect Transistor/quasi Reference Electrode apparatus.
2. Description of the Prior Art
The initial concept of Ion Sensitive Field Effect Transistor (ISFET) was proposed by P. Bergveld in 1970 first. It has been improved and developed continuously. Since the development of Ion Sensitive Field Effect Transistor, the specific sensitive membrane material had been developed for many pH Ion Sensitive Field Effect Transistors (pH-ISFET). This material has high sensitivity and will not be interfered by non-ideal effect. In addition, due to the structure of Ion Sensitive Field Effect Transistor is similar to the structure of Metal-Oxide-Semiconductor Field Effect Transistor, the Complementary Metal Oxide Semiconductor (CMOS) process can be used to manufacture the Ion Sensitive Field Effect Transistor and to be microminiaturized. The Ion Sensitive Field Effect Transistor has very high biocompatibility, thus it is widely applied to the fields, such as biochip, human body inspection etc.
However, there still is a big problem to be solved in the manufacturing of micro Ion Sensitive Field Effect Transistor system. This problem relates to the reference electrode. At present, all Ion Sensitive Field Effect Transistor systems must use the conventional glass reference electrode (such as Ag/AgCl or calomel electrode) to provide the stable potential. However, the electrode stability and service life of these reference electrodes will be reduced due to the decrease of internal ion exchange solution under the microminiaturization process. Thus, it is very difficult to microminiaturize the reference electrode. It has seriously restrained the application and development of Ion Sensitive Field Effect Transistor on the biomedical field and human body inspection and diagnosis.
Matsuo proposed the Reference Field Effect Transistor (REFET) apparatus first in 1978. At present, the research of this element can be divided into three categories mainly:                1. Add an ion-blocking layer on the original inorganic membrane, in order to reduce the number of ionic bonding on the surface of sensitive membrane.        2. Employ the polymer to form an ion-blocking membrane. However, it is limited to the problem of membrane thickness. The membrane thickness must be increased, in order to improve the defect of membrane pore. When the membrane thickness is increased, the transconductance of element will be decayed, which will lead to the mismatch of system operation. In addition, this kind of membrane has the stability and high sensitivity problems.        3. Employ the polymer to form an ion-unblocking membrane. It can solve the above-mentioned decay problem of transconductance. It is also the most mature and stable membrane with low sensitivity at present. Even so, the service life of this kind of membrane is still short.        
The development of organic Reference Field Effect Transistor (using PVC membrane) is mature, where the sensitivity can be reduced to about 1 mV/pH to 2 mV/pH at present. However, there organic membrane and the semiconductor element still are the problems of complicated process and incompatible structure. Thus, the development of inorganic Reference Field Effect Transistor not only can simplify the process, but also can be compatible with the CMOS process completely and can avoid the transconductance decay of organic membrane. It will be an innovative development on the fields of Reference Field Effect Transistor and Ion Sensitive Field Effect Transistor.
The structure of Reference Field Effect Transistor and Ion Sensitive Field Effect Transistor are very similar. The main difference is that the Ion Sensitive Field Effect Transistor is very sensitive to the target ion (such as hydrogen ion, sodium ion, and potassium ion), but the Reference Field Effect Transistor is less sensitive to the target ion. The Ion Sensitive Field Effect Transistor/Reference Field Effect Transistor should have the quasi Reference Electrode (qRE) to provide the bias voltage to the sensing system to form the circuit. The differential amplifier is used to differentiate the output voltage of quasi Reference Electrode. The final obtained output voltage is the output ion concentration of Ion Sensitive Field Effect Transistor/Reference Field Effect Transistor system. The influence of voltage between unstable solid and liquid interface can be offset by the differential amplifier. From the above-mentioned description, the Reference Field Effect Transistor with low ion sensitivity can be formed as the combination of Ion Sensitive Field Effect Transistor/Reference Field Effect Transistor/quasi Reference Electrode (IS FET/REFET/qRE).
In the conventional Ion Sensitive Field Effect Transistor, due to the part of reference electrode is difficult to be microminiaturized and integrated into the integrated circuit (IC), thus the Ion Sensitive Field Effect Transistor/Reference Field Effect Transistor system has been proposed. When the ion sensitivity is reduced, it is necessary to be dependent on a layer of extra organic membrane. Thus, the complexity of process is increased, and the service life is also reduced.
From the above-mentioned reason, it is known that the development of Reference Field Effect Transistor is paid more attention day by day. In order to respond the future demand, it is necessary to develop relevant technology of inorganic Reference Field Effect Transistor apparatus, in order to reduce the cost of operation manpower and manufacturing time, and reach the purpose of energy conservation and carbon reduction effectively.